A process for preparing lactose-free skimmed, partially skimmed and whole milk is proposed in which: (1) the milk is pasteurized and skimmed; (2) the skimmed milk is hydrolyzed enzymatically by a lactase; (3) the hydrolyzed skimmed milk is microfiltrated to obtain an ultrafiltration retentate (RMF) and a microfiltration permeate (PMF); (4) the PMF is ultrafiltrated and an ultrafiltration retentate (RUF) and an ultrafiltration permeate (PUF) are obtained, 5) the PUF is nanofiltrated and a first nanofiltration retentate (RNF1) and a first nanofiltration permeate (PNF1) are obtained, 6) the PNF1 is nanofiltrated to obtain a second nanofiltration retentate (RNF2) and a second nanofiltration permeate (PNF2), and 7) final step: lactose-free milk is obtained by mixing one or more of the fractions deriving from one or more of the previous steps.